This invention relates to gear oils for use in heavy duty (HD) axle and transmission applications. More particularly, this invention relates to extended drain, thermally stable gear oils.
Gear oils are different from other lubricants as the conditions experienced in manual transmissions and axles are extreme. One major difference in the composition of gear oils from other lubricants is the presence of extreme pressure (EP) agents. These EP agents often contain high levels of sulfur which are unacceptable to other lubricants due to oxidation problems.
There are also different performance requirements for specific gear oils directed for use in manual transmissions and final reduction gear sets. For example, final reduction gear sets require higher EP operation conditions. There presently exists a need for a dual-purpose gear oil for both manual transmissions and final reduction gear sets to economize maintenance.
Japanese laid-open patent (JP 328084) “Hino” describes an automotive gear oil composition for both manual transmissions and final reduction gear sets. Hino discloses three specific antiwear additives, including phosphites, along with an alkyl-t-butyl trisulfide EP additive. While alkyl-t-butyl trisulfides are thermally stable, they lack sufficient EP performance and do not provide acceptable GL-5 shock performance without going to very high treat rates or adding additional EP components. Furthermore, the phosphites can react with trisulfides to form undesirable odorous mercaptan by-products. Finally, phosphites do not provide sufficient break-in performance for high temperature wear protection.